[R] help with using last observation carried forward analysis for a clinical trial please

[David Winsemius]

On Oct 8, 2011, at 8:43 PM, maspitze wrote:

> Hi,
> I have a series of id's with multiple visits and questionnaire  
> scores.  This
> is a clinical trial that will be analyzed using the last observation  
> carried
> forward method.  In other words, in order to comply with intent to  
> treat
> analysis when many subjects withdraw, data points for the last visit  
> must be
> generated and filled in with the last observation.  The ultimate  
> goal is to
> tabulate the difference in qustionnaires between the start of the  
> trial and
> the end of trial.  I am lost at about how to do this.
> Each subject had multiple visits, up to 13.  In general, they took a
> questionnaire at each visit.  However, if a questionnaire was not  
> completed
> or the visit is missing, the data point does not exist.  To explain, I
> created a table as analogy.
>
> My goal is to take something that looks like the following:
>
> ID	Visit	score
> 1	1	10
> 2	1	12
> 2	3	15
> 3	1	1
> 3	2	6
> 4	1	16
> 4	2	1
> 4	3	7
> 4	4	17
>
> I think I then need to change to this in order to perfrom locf in zoo:
> ID	Visit	score
> 1	1	10
> 1	2	na
> 1	3	na
> 1	4	na
> 2	1	12
> 2	2	na
> 2	3	15
> 2	4	na
> 3	1	1
> 3	2	6
> 3	3	na
> 3	4	na
> 4	1	16
> 4	2	1
> 4	3	7
> 4	4	17
>

require(zoo)
dat2 <- merge(data.frame(ID=rep(1:4, each=4), Visit=rep(1:4, 4)), dat,  
all.x=TRUE)
  dat2$Vscr.fill <- na.locf(dat2$score)
  dat2
  lapply(split(dat2, dat2$ID), function(x) x$Vscr.fill[4]-x 
$Vscr.fill[1] )
#########
$`1`
[1] 0

$`2`
[1] 3

$`3`
[1] 5

$`4`
[1] 1

-- 
David


> then change to:
> ID	Visit	score
> 1	1	10
> 1	2	10
> 1	3	10
> 1	4	10
> 2	1	12
> 2	2	12
> 2	3	15
> 2	4	15
> 3	1	1
> 3	2	6
> 3	3	6
> 3	4	6
> 4	1	16
> 4	2	1
> 4	3	7
> 4	4	17
>
> I would then like to take visit 4 and subtract visit 1 to create the
> difference in the questionnaire scores during the clinical trial.  I  
> will
> then compare this score by t.test between placebo and drug groups.
>
> Would anyone please have some guidance about how to do this in r?  I  
> would
> be grateful for assistance.
> Regards, Matt
>
>
> --
> View this message in context: http://r.789695.n4.nabble.com/help-with-using-last-observation-carried-forward-analysis-for-a-clinical-trial-please-tp3886396p3886396.html
> Sent from the R help mailing list archive at Nabble.com.
>
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David Winsemius, MD
West Hartford, CT